The present invention is directed to an improved process for the manufacture of acrylic acid. Presently, acrylic acid is produced by a two-step process. Propylene is first oxidized to acrolein over a mixed metal oxide catalyst comprising iron, bismuth and molybdenum promoted with suitable elements, and the acrolein is further oxidized to acrylic acid over a second catalyst in a separate reactor. Typically, catalysts containing oxides of iron, bismuth and molybdenum promoted with suitable elements are readily available for the selective oxidation of the propylene to acrolein (i.e. this first step in the two-step process in the manufacture of acrylic acid). Examples of suitable types of catalysts for this first step can be found in U.S. Pat. 4,162,234 and 4,280,929 assigned to the assignee of the present application.
In the second step of the two-step process acrolein is oxidized over the second catalyst to acrylic acid. It is always the case that the selectivity of the acrolein to acrylic acid is below 100%. However, the acrylic acid that is formed in the first step of the two-step process passes through the second reactor with no decomposition. Therefore, it is advantageous to use catalysts that produce substantially larger amounts of acrylic acid during the oxidation of the propylene to acrolein in the first reactor, thereby getting higher yields of acrylic acid in the two-step process.
In related patent application U.S. Ser. No. 08/923,878 filed Sep. 2, 1997, and assigned to the assignee of the present invention, there is a disclosure of a novel catalyst useful in the manufacture of acrylonitrile and hydrogen cyanide. The catalyst was specifically disclosed as containing a mixed metal oxide of iron, molybdenum and bismuth promoted with various metals and useful in the manufacture of acrylonitrile with substantially higher yields of co-product hydrogen cyanide. It is the discovery of the instant application that the catalyst of co-pending application 08/923,878 can not only be used in the first step of the two-step process for the manufacture of acrylic acid, but results in unexpected high yield of acrylic acid during the first step of the process. This high yield of acrylic acid in the first step leads to a higher yield of acrylic acid overall being achieved in the two-step process.